Microplastics (MPs) have been recently recognized as posing a risk to human health. The adverse health effects of MP exposure have been recently reported, especially via the oral exposure route. The present study investigated whether subacute (4 week) exposure to polyethylene (PE) or polytetrafluorethylene (PTFE) MPs via gastric intubation caused immunotoxicity. Two different sizes of PE MPs (6.2 or 27.2 μm) and PTFE MPs (6.0 or 30.5 μm) were administered to 6-week-old mice of both sexes at 0 (corn oil vehicle control), 500, 1000, or 2000 mg/kg/day (n = 4/group). No significant differences were observed between groups in the major thymic or splenic immune cell populations, including thymic CD4+, CD8+, CD4+/CD8+ T lymphocytes, and splenic helper T cells, cytotoxic T cells, and B cells. The ratio of interferon-gamma (IFNγ) to interleukin-4 (IL-4) in culture supernatants from polyclonally activated splenic mononuclear cells ex vivo (48 h) was dose-dependently decreased in female mice that received small- and large-size PTFE MPs. The IFNγ/IL-4 ratio was also decreased in the female mice dosed with large-size PE MPs. The serum IgG2a/IgG1 ratio was dose-dependently increased in male and female animals dosed with small-size PE MPs, in female animals dosed with large-size PTFE MPs, and in male animals dosed with small-size PTFE MPs. The present study implies that immune functions could be affected in animals exposed to MPs via gastric intubation. These effects are dependent on MP size, MP dose, MP polymer type, and mouse sex. Further investigations with longer exposure periods could be necessary to more clearly define the immunotoxic effects of MPs. The online version contains supplementary material available at 10.1007/s43188-023-00172-6.